If you really want an answer, you should change the title to indicate the nature of the question to get the right people to read your question. I'm no expert but my understanding was that a green egg was a brown egg with a blue overlay, so your chicken just missed the overlay today. I'm sure there will be someone along to prove me wrong on this, but that's ok, I'm willing to learn.

If a hen can lay a shell less egg, or a very thin shelled egg (meaning it passed through too quickly to get the proper amount of shell laid down), it just makes sense that a very thin shelled egg in a chicken that usually lays green eggs would result in a brown egg.

That is, if you are sure you don't have any brown egg layers in your flock, or something more obvious like that...